The present invention generally relates to the production of woven glass fabrics and particularly to the drying of woven glass fabrics for use as a substrate for printed circuit boards.
Laminated circuit boards are used widely in sensitive electronic equipment such as computers and communications equipment. Circuit boards, generally, are multi-layer composites of thermosetting polymers and suitable reinforcing materials. One suitable reinforcing material is woven glass fabric.
Such reinforcing fabrics are extremely fine, light weight and very fragile. During processing, the fabric is particularly susceptible to the presence of any residue or particulate matter that, if conductive, may cause performance malfunctions (such as short circuits) in the finished printed circuit boards. Due to such sensitivity, high standards of cleanliness and quality are imposed. Fabrics commonly are rejected if the fabrics are contaminated with any foreign particles, whether conductive or not.
Heretofore, such fabrics have been dried using hot air, contact heat, or infrared radiation. Such drying techniques are acceptable for ordinary applications in which deposits of some foreign particulate matter will not adversely affect the product or may be washed out after subsequent operations. However, these techniques have proved to be a problem where the fabric is used in the manufacture of printed circuit boards. Hot air, contact heat, and/or infrared radiation techniques are prone to leave fine particles that may contaminate the woven glass fabric. Hot air introduces a foreign fluid stream that is riddled with particulate matter. Contact heat can cause migration from one face of the fabric to another and results in uneven and non-uniform drying. Infrared radiation cannot be controlled sufficiently; therefore a mechanical shutter system must be used to prevent excessive heating of the fabric. Tile shutter system introduces rust, dirt, and other particulate matter to the fabric environment. Further, fiberglass fabrics are fragile and are often damaged during these harsh drying processes.
There exists a need for a more uniform and controlled drying process that achieves the necessary cleanliness and sensitivity required for fabrics to be used in laminates for electronics.
The present invention is a process for the manufacture of woven glass fabric to be used in sensitive electronic equipment. The process adopts a drying technique utilizing dielectric or radio frequency heating.
Radio frequency drying involves the heating of electrically nonconducting materials by a rapidly varying electromagnetic field. While radio frequency drying (RF drying) has been used to dry fabrics, the technique generally has been used in shrinkage control systems.
With respect to cleaner drying systems for sensitive fabrics such as fiberglass fabrics for printed circuit board laminates, however, the use of radio frequency energy is unknown. Generally, RF drying processes are not as cost-effective as other drying processes such as hot air, contact heat or infrared radiation. However, RF drying is substantially cleaner and produces a higher quality substrate fabric for electronic equipment. With RF drying there is no pollution from gas or oil fired burners or electric heating elements. The reduced air volume that is required with RF drying reduces air-based contaminants. Further, the RF oven itself is constructed of aluminum thereby eliminating the rust particles present in conventional dryer baffles and ductwork.
It is an object of the present invention to produce a cleaner oven glass fabric well suited to use as a reinforcing laminate for electronic applications. An added feature of drying glass fabrics by RF is the increased control over the drying process. The use of a controllable RF dryer eliminates the need for accumulators in the process. Accumulators arc used in conventional fabric drying operations to facilitate continuous running and avoid stopping the dryer. Neither entrance nor exit accumulators are necessary with RF dryers. Accumulators are a chronic source of fabric distortion due to alignment difficulties. It is easier to handle fragile, lightweight fabrics when the accumulators are removed from the process. Moreover, the associated problems of accumulators, such as alignment, are eliminated. The RF dryer can be quickly stopped without accumulators to avoid damage to the glass woven fabric.